Satire
Sure, there is some seriously tragic stuff happening in this book. The body count adds up pretty fast when Arrowsmith takes his Caribbean vacation, er, plague vaccination expedition. But when taken as a whole, Arrowsmith shines with Sinclair Lewis' patented satire.
No matter what he's talking about, Lewis has the ability to focus his eagle eye on human pettiness and hypocrisy. Take Dr. Pickerbaugh, for example, who says, "No matter what methods we use, if we can get people to have more fresh air and cleaner yards and less alcohol, we're justified" (21.2.6). Pickerbaugh is a guy who believes in good ol' fashioned Christian morality. But he has no problem lying to people as long as people behave the way he thinks they should.
Sinclair Lewis is directing his white-hot sarcasm towards the fraught world of science. Through satire, Lewis illuminates the nepotism, politics and small-mindedness of a field that is supposed to be working toward the greater good. But Lewis is an equal-opportunity ironist, and skewers superiority complexes right along with stupidity.
There is no character in this book that Lewis doesn't view through a satirical lens, including our hero Martin Arrowsmith. Arrowsmith is introduced to us with the written equivalent of a flashing neon "smug" sign on his head: "On his first day in medical school, Martin Arrowsmith was in a high state of superiority" (2.3.1). Sure, we like him, but we realize that he's a little too big for his britches.